Report of Ting-Wei's 1st secondment @IFREMER
TING-WEI WU (ESR7) | @ IFREMER BREST, FRANCE (November-December 2018)
During the last two months of the year 2018, I would like to thank Dr. Antonio Cattaneo to host my stay at the Ifremer (Center of Brest/Plouzané). This was the first time that I did a secondment, and it had been inspirational! A new project to study about the “seismic strengthening” behavior of saturated Ottawa Sand 20/30 was initialized from this secondment.
Seismic strengthening is the increase of sediment’s strength due to densification after earthquake events that are not strong enough to cause failure. This hypothesis can be simulated in the laboratory by measuring the increase of undrained shear strength after cyclic loadings and drainage. The study of undrained shear strength is important because it is directly relevant to slope stability analysis. The outcome will, for example, improve the understanding of the initiation (or no initiation) mechanism of submarine landslides under earthquakes.
Together with my co-supervisor Dr. Nabil Sultan, we developed a testing procedure that utilized load-controlled undrained cyclic triaxial tests in anisotropic conditions with different initial relative densities and mean effective stresses. After imposing seismic loadings without making a specimen fail and subsequently draining the specimen with sufficient time to represent drainage after an earthquake, the increase of its shear resistance could be measured. We aimed to demonstrate the threshold between failure and strengthening under cyclic loadings, document the density increase after drainage, and eventually discover the trends of seismic strengthening of Ottawa Sand 20/30.
It was very technically demanding to conduct cyclic triaxial tests: from the sample preparation, saturation, to the control of undrained and drained conditions, all the steps needed to be handled with a lot of care in order to get good data quality. 49 tests were done in these two months (not all of them were successful). Preliminary findings will be demonstrated in the EGU General Assembly in 2019.
Since this was my first project to do triaxial tests, I was not skilled at all in the beginning. I am extremely grateful to Dr. Nabil Sultan for his frequent support of my experiments and theory with his patience and expert knowledge. He took a lot of time out from his busy schedule to define the project with me and supported me in overcoming numerous obstacles I had faced through this research. I could not imagine having a more excellent supervisor in the field of geotechnical engineering for the progress of this project. “You are here to learn!” was a nice thing he said to me, especially encouraging at the times when I felt kind of defeated because many of my early tests were not successful.
A special gratitude goes to Dr. Sebastien Garziglia for the assistance in calibrating the sensors as well as his advice to make better saturation of my samples, which significantly improved the data quality of my research.
I would also like to thank many doctoral students and technicians Arthur Blouin, Mickaël Rovere, Mickaël Roudaut, Shray Badhani, Farah Taleb, Pauline Dupont, Xudong Wang… for the technical assistance of the machines. It was fantastic to have the opportunity to work part of my research at the Ifremer with you. What a cracking place to work!
These two months were short but were probably the most satisfying period of my PhD research so far. Thank you for all your encouragement, and I look forward to working with you again!